1. Field of the Invention
The present invention relates to a pressure sensitive adhesive for polarizing plates, a polarizing plate having a pressure sensitive adhesive, a process for the production of the polarizing plate, an optical film using the polarizing plate having a pressure sensitive adhesive and a process for the production of the optical film. More particularly, the present invention pertains to a pressure sensitive adhesive for polarizing plates which is preferably applied to polarizing plates, particularly to a polarizing plate integrated with a visual field angle-expansion film or the like, or a polarizing plate that is laminated with a phase difference plate, and which is characterized in that it is capable of sticking with high durability, the polarizing plate to a liquid crystal cell or a phase difference plate and, that a liquid crystal display unit to be obtained therefrom is less liable to cause light leakage even in an environment of high temperature and high humidity; to a polarizing plate having a pressure sensitive adhesive; to a process for producing the polarizing plate thereof; an optical film using the polarizing plate having the pressure sensitive adhesive; and a process for producing the optical film thereof.
2. Description of the Related Arts
There have heretofore been frequently caused such unfavorable situations that in the case of sticking a sheet composed of an organic material on an adherend such as glass, ceramics or a metal via a pressure sensitive adhesive, the sheet peels off at an end and/or brings about lifting or relief accompanying the lapse of time.
In order to solve the above-mentioned unfavorable situations, there is generally employed a strongly tacky adhesive material which is enhanced in pressure sensitive adhesive performance by increasing the molecular weight of the components constituting the pressure sensitive adhesive or enhancing crosslinking density. However, the use of the strongly tacky adhesive material, although enhances the holding power, makes the pressure sensitive adhesive incapable of no more following shrinkage and swelling of the sheet composed of an organic material by occurring under high temperature and high humidity conditions, thereby contributing to the occurrence of various troubles.
In this connection, some of optical parts are used by sticking a polarizing plate on a surface thereof, and are typically exemplified by a liquid crystal cell of a liquid crystal display unit (LCD). In the following, some description will be given of the liquid crystal display unit with reference to FIG. 1.
Liquid crystal cell 13 generally has such a structure that two transparent electrode substrates each forming an orientation layer are arranged so that the substrates are spaced at prescribed intervals with the orientation layers being located in the interior side, the perimeters of the substrates are sealed, a liquid crystal material is put in the above-mentioned intervals and polarizing plates 11, 11 are each placed on the two transparent electrode substrates via a pressure sensitive adhesive 12, 12. The above-mentioned polarizing plate generally comprises polarization films having three layer constitution in which optical isotropic films, for instance, triacetyl cellulose (TAC) films are stuck on both the sides of polyvinyl alcohol based polarizer. A pressure sensitive adhesive layer is formed on one side of the polarizing plate for the purpose of sticking the polarizing plate on an optical part such as a liquid crystal cell. As illustrated in a schematic drawing shown in FIG. 2, a phase difference plate 24 is installed as the case may be, between a polarizing plate 21 and a liquid crystal cell 23 via pressure sensitive adhesive 22, 25 in order to improve visual field angle characteristics.
In the case where the polarizing plate of the foregoing constitution is stuck on an optical part such as a liquid crystal cell, or the polarizing plate and a phase difference plate are stuck thereon, the resultant multi-layer structure of different materials brings about poor dimensional stability in terms of material properties, and particularly in an environment of high temperature and high humidity, gives rise to large dimensional variation due to shrinkage and swelling. Since the above-stated strongly tacky adhesive material is generally used as the pressure sensitive adhesive in the polarizing plate, it is possible to suppress lifting and peeling off which accompany dimensional variation of the polarizing plate, however it is impossible to absorb, in a pressure sensitive adhesive layer, the stress accompanying the dimensional variation of the polarizing plate, whereby the residual stress in the polarizing plate is made non-uniform. Consequently, problems are raised in that a TN cell and an STN cell are liable to appear so-called light leakage and color unevenness, respectively.
In order to solve the above-described problems, there is disclosed technology of imparting stress relaxation properties by adding, for instance, a low molecular weight body such as a plasticizer to the pressure sensitive adhesive so as to moderately soften the same (for instance, refer to Patent Literatures No. 1). Nevertheless, the addition of a low molecular weight body is responsible for adherend pollution in the case of peeling off the polarizing plate, besides leads to lowered holding power, and is more liable to give rise to lifting and peeling off that are due to the lapse of time.
Accordingly it has been the major subject to reconcile adhesion durability and light leakage preventability in a liquid crystal cell and the like.    Patent Literature No. 1: Japanese Patent Publication No. 3272921